The present invention relates in general to radio frequency (RF) communications systems and, in particular, to an RF repeater with a delay to improve hard handoff performance between cells and method therefor.
Throughout the world, certain RF bands have been allocated for various types of communications, including personal communications system (PCS), cellular, and other mobile applications. In the United States, the Federal Communications Commission (FCC) has allocated frequency bands in the range of 824-849 and 869-894 MHz; and 1850-1910 and 1930-1990 MHz for such applications. Currently, the 824-849 and 869-894 MHz bands are used for mobile cellular communications and the 1850-1910 and 1930-1990 MHz bands are used for PCS applications. Other countries utilize their own frequency spectrum for such applications.
Within each service provider""s allocated band and geographic area (usually there are two or more service providers in a geographic area using a different portion of the spectrum), the service provider may utilize any type of technology including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), or combination thereof.
Frequency division multiple access (FDMA) technology utilizes narrow and discrete channels within the frequency band. Different subscriber stations are assigned different frequency channels. Interference to and from adjacent channels is limited by the use of bandpass filters which pass the signal energy within the narrow channels while rejecting signals having other frequencies. The United States cellular system (AMPS) divides the allocated spectrum into 30 KHz bandwidth channels and uses FM modulation.
Time division multiple access (TDMA) technology also utilizes narrow and discrete channels within the frequency band. However, each channel is further divided into time slots in the time domain. This results in multiple users on the same frequency channel and increases the number of users per given channel.
Unlike FDMA or TDMA, code division multiple access (CDMA) involves multiple users simultaneously sharing a channel having a relatively wide bandwidth. United States CDMA standards (IS-95) currently specify a CDMA channel having a bandwidth of 1.25 MHz. In CDMA, a large number of signals share the same frequency spectrum. Each signal consists of a different pseudorandom binary sequence that modulates a carrier signal (at the center frequency of the channel""s spectrum). This spreads the spectrum of the waveform over the entire channel bandwidth. Use of CDMA technology allows for a larger number of signals than that used in FDMA or TDMA within the same amount of frequency spectrum and geographic area. Use of CDMA technology by service providers is expected to grow due to its increased traffic capabilities, digital technology and security. The PCS and cellular systems were initially designed and deployed with FDMA or TDMA technology (or a specific standard such as AMPS). Because no additional frequency spectrum has been allocated by the FCC in the cellular band, cellular band service providers desiring to use CDMA technology are now integrating CDMA technology into existing systems and must utilize the same frequencies currently allocated.
Typically, service providers in PCS, cellular and other mobile applications divide the particular geographic region in which they are operating into xe2x80x9ccellsxe2x80x9d. This concept is well-known in the industry. Each cell contains a base station (including a transmitter and receiver) and services subscriber users within the boundaries of the cell. Each service provider is free to design its own coverage system including the locations and sizes of its cells.
In FDMA and TDMA, adjacent cells must use channels having different frequencies to avoid interference (a re-use factor of K=7 is traditionally used). When a subscriber station moves from one cell to an adjacent cell, a new communications channel between the subscriber station and the base station in the adjacent cell must be established. The communications channel between the subscriber station and the base station in the original cell is then terminated. This process is known as a xe2x80x9chand-offxe2x80x9d, and is more particularly described as a xe2x80x9chardxe2x80x9d hand-off because the process requires switching frequency.
In CDMA technology, however, each cell may use all or any portion of the frequency spectrum allocated to the service provider. Therefore, the same frequency channel (i.e., f1) may be used in adjacent cells. This increases the number of users within a particular cell and geographic area.
One of the fundamental features of the CDMA cellular technology is the use of a xe2x80x9csoftxe2x80x9d hand-off process. A soft hand-off occurs when a subscriber station travels from one cell to an adjacent cell with both cells operating at the same channel frequency. In the soft hand-off process, a subscriber station establishes a communications link to an adjacent cell before breaking off communications with the original cell. In most cases, the subscriber station maintains its link for a period of time to both the base station of the original cell and the base station of the adjacent cell. In some cases, the subscriber station may be in the soft hand-off process and communicate with more than two base stations. For the soft handoff to work, it is necessary that the two base stations (or more) involved use the same channel frequency.
A single CDMA channel frequency may not be sufficient to provide service in some areas of the overall service area. Where the need for service is high, the number of channel frequencies must also increase. If the additional channel frequencies cover all cells of the service area, then the subscriber station will keep the same channel frequency while it moves from cell to cell within the service area. However, adding additional channel frequencies to each cell may not be desirable because some cells may not need the increased capacity. Accordingly, some cells that experience high traffic may require one or more additional channel frequencies. This results in a condition where a subscriber station may be required to switch its channel frequency when moving from one cell (i.e. using channel frequencies f1 and f2) to an adjacent cell (i.e. using only channel frequency f1). When the subscriber station is communicating with the cell using channel frequency f2 and moves to the adjacent cell, the soft hand-off process is no longer viable since the subscriber station cannot transmit simultaneously at the two different frequencies needed to communicate with the two base stations. When the soft hand-off process cannot be used, the subscriber station must break from one base station (operating at f2), switch frequency to the new channel frequency (f1), and establish the link with a new base station (operating at f1) operating within the original cell. This process is known as a xe2x80x9chardxe2x80x9d hand-off. After the hard hand-off is completed, then the soft hand-off process may be used when the subscriber station (now using channel frequency f1) moves from the original cell to the adjacent cell (using channel frequency f1).
The problem with this type of hard hand-off is that the effective coverage area of the CDMA source cell is reduced due to several factors. First, the hard hand-off process may take up to five (5) seconds to complete. Assuming that the subscriber station""s velocity may be up to seventy (70) m.p.h. and directly away from the base station of the source cell, then the subscriber station may travel approximately 156 meters from the initiation of the hard hand-off process until the process is completed. Second, adding the round trip delay (RTD) uncertainty of about 244 meters, then the subscriber station must be approximately 400 meters or more away from the edge of the cell coverage area when the hard hand-off process is triggered.
In addition, in the case as described above where the subscriber station switches from the channel f2 to the channel f1 within the source cell (hard hand-off) and then uses a soft hand-off to the adjacent cell, there is no soft hand-off gain for the subscriber station that is going to hard hand-off to the collocated base station operating at f1. As recalled earlier, the soft hand-off process results in the subscriber station communicating simultaneously with at least two cells (the source cell and the likely destination cell) and perhaps other cells (additional cells near the subscriber station). Near the cell boundary the subscriber station is receiving separate, but identical, communication signals from the two or more cells resulting in a multi-path increase in the strength of the received signal. Therefore, the forward link Eb/No requirement is lower allowing a reduction in power of the forward link signals for the same coverage area. However, when the subscriber is operating at f2, there is no soft hand-off gain because the subscriber will not be experiencing any multi-path increase in the signal strength. This results in a higher forward link Eb/No requirement and lowers the effective coverage area of the source cell. The amount of reduction depends on a number of factors and ranges from a low of about 0 dB to as high as 15 dB with an average estimate of about 5 dB. This means that the effective coverage area of the source cell (operating at f2) is roughly about 5 dB smaller.
The combination of a hard hand-off to a collocated base station within the source cell and then a soft hand-off to the adjacent cell is one method of lessening some of the foregoing problems, however, it has several disadvantages. One disadvantage is the increased cost of an additional and collocated base station in the source cell. This method currently provides for the subscriber station to switch to a collocated base station (BTS).
Another disadvantage is that the source cell BTS (after achieving a hard handoff from the other source cell BTS) may not be found and detected by the subscriber station resulting in hand-off failure and call drops. This problem is aggravated in multi-sector cells. There are a number of adjacent cells and/or sectors (within a cell) but only the collocated BTS is the one which should be used as the destination BTS. If this BTS is not detected by the subscriber station, the call will drop and the hand-off fails. Furthermore, in order to achieve customer satisfaction and normally complete handoffs, the collocated source cell BTS must save a certain forward link transmit power and resources to allow switching to its channel frequency so the subscriber stations can perform the hand-off process using that particular channel frequency. All of this causes a heavy penalty on the network and its capacity.
Another disadvantage of the foregoing method is that the destination BTS must allocate a large amount of forward link transmit power to the subscriber station because it is near the cell edge of the source cell (and relatively far away from the BTSs) where the hard hand-off is triggered. Because of the distance between the destination BTS and the subscriber station, and in order to communicate, a relatively large amount of forward link transmit power to the subscriber station is required. This decreases the forward link capacity of the BTS to which the subscriber station hands-off.
In another case, a provider""s service may be deployed in a fashion (no ubiquitous channel frequency within the source and destination cell) such that a subscriber station when moving from one cell operating only at one channel frequency (i.e., f2) to an adjacent cell operating at a different channel frequency (i.e., f1) must switch frequency. In this case, the effective coverage area of the source cell is also negatively impacted as described above.
To eliminate some of these disadvantages, one meth od utilizes an additional collocated base station. The subscriber station hands off to the collocated base station through a hard hand-off process. One method that does not require the use of the additional collocated base station uses beacons or the like. This method may lessen some of the foregoing problems, however, it is expensive and also suffers from some of the same disadvantages as described above, such as decreased forward link capacity, the uncertainties in finding the destination base station and wasteful allocation of resources.
In addition to the CDMA-to-CDMA hard hand-off process, a hard hand-off is also used when a subscriber station switches from CDMA to AMPS (FDMA or TDMA). This requires not only switching frequency, but also the mechanism of communication. Further, a hard hand-off is also used in all FDMA or TDMA when the subscriber station is required to switch frequencies when moving from one cell to another cell.
As clearly illustrated, the current methods of CDMA hard hand-off adversely impact the CDMA hard hand-off performance of the source cell when a subscriber station requires a hard hand-off from the source base station to the destination collocated base station. This adverse impact effectively reduces the effective coverage area of the source cell. Cells in high traffic urban areas are generally small (i.e., between 1 and 2 kilometers in diameter). The reduction in cell coverage area caused by the hard hand-off process does not leave much coverage area for use in the source cell. This increases the cost to provide service.
Accordingly, there exists a need for a system and method that improves CDMA hard hand-off performance from CDMA cell to CDMA cell, or from CDMA cell to another cell that uses different technology (AMPS, GSM, FDMA, TDMA, etc.).
According to the present invention, there is provided an RF repeater for use in a cell to improve hard hand-off performance. The RF repeater includes an input/output terminal for receiving a first signal from a subscriber station. A predetermined amount of delay is added to the received first signal and the delayed signal is output for transmission to a base station. In another aspect of the present invention, there is provided an RF repeater that includes an input/output terminal for receiving a first signal from a base station. A predetermined amount of delay is added to the received first signal and the delayed signal is output for transmission to a subscriber station.
The added delay distinguishes the RF repeater signal from the signal received directly from the subscriber station and allows the base station to determine the approximate location of the subscriber station (i.e., within the coverage area of the RF repeater and near the cell boundary for initiation of the hard hand-off process).
In another embodiment of the present invention, there is provided an apparatus for delaying an RF signal between a subscriber station and a base station for improving hard hand-off performance between a first cell and a second cell. The apparatus includes an RF repeater, operable with a base station within a first cell, for receiving an RF signal from a subscriber station and for transmitting the RF signal to the base station. A predetermined amount of delay is added to the RF signal (in the reverse path). In another aspect, the RF repeater receives an RF signal from a base station and transmits the RF signal to a subscriber station. A predetermined amount of delay is added to the RF signal (in the forward path).
In another aspect of the present invention, there is provided a communications cell having a base station emitting a base station signal for communicating with a subscriber station within a predetermined geographic area. An RF repeater operable with the base station and located proximate a boundary of the predetermined geographic area is included and has a terminal for receiving a first signal from the subscriber station. A predetermined delay is added to the first signal and the delayed signal is output for transmission to the base station.
In yet another aspect of the present invention, a method is provided that improves hard hand-off performance from a first cell to a second cell. An RF repeater operable with a base station receives a first signal from a subscriber station. The received first signal is delayed by a predetermined amount of time and the delayed first signal is output for transmission to the base station. Similarly, the delay may be added to the forward path.
In another aspect of the invention, there is provided a method of defining the location of a subscriber station located within a cell or near the boundary of the cell. The method includes the step of receiving at a radio frequency (RF) repeater operable with a base station of the cell a first signal from a subscriber station. A predetermined amount of delay is added to the first signal and output for transmission to the base station. The base station receives the delayed first signal from the RF repeater and determines the location of the subscriber station from the received delayed first signal.